Tongan Law Lord, Ramsay Dalgety, is preparing a civil claim for malicious prosecution after a perjury indictment against him was thrown out on Friday following a pre-trial application.

Lord Dalgety had been charged over evidence he'd given to the Commission in Inquiry into the sinking of the inter island ferry, Princess Ashika.

But Justice Charles Cato discharged the case ruling saying there was insufficient evidence of perjury.

Lord Dalgety's lawyer, Kahu Barron-Afeaki, says his client had come under a sustained attack at the Commission of Inquiry.

He says he believes the Commission had a clear intention to discredit Lord Dalgety.

"Sometimes royal commissions have to undergo this experience with their wide sweeping powers of witch-hunting and I think they made some pre-judgements and without going through the proper enquiry process and they were wrong. All the accusations against him were refuted."